Previous investigations have described both antigen-specific and antigen-nonspecific T cell-mediated regulatoary mechanisms capable of suppressing the in vitro generation of cell-mediated cytotoxicity. Further experiments have been directed at determining the cellular interactions involved in the in vitro generation of suppressor cells, and at analyzing genetic differences in the abilities of different inbred mouse strains to generate suppressor cells. Most recently, monoclonal anti-Lyt reagents have been employed to characterize the T cells responding in cell-mediated cytotoxicity as Lyt 1+2+. Two regulatory pathways have been described, one mediated by Lyt 1+2- T cells which nonspecifically suppresses the generation of cell-mediated cytotoxicity, and one requiring Lyt 1+2+ T cells which suppresses in an antigen-specific fashion.